Ryan... said:
I was averaging almost 25mpg with 80% hwy driving in my 540i. Ran like a top, 128k miles. I imagine in a lighter chassis (like the e36) it wouldn't be unreasonable to get 26-27, which isn't much worse than your typical e36 325i gets anyway.
the m60s are fine when cruising at constant speeds, but throw in high revs/stop and go and it all goes to hell. no one is going to swap an m60 into an e30 and then use it on for 80% highway driving. driving enthusiastically with the 6banger is still going to get you 22-24mpg, but driving the m60 that way is going to be 16-18mpg.
my point is probably moot since im sure if you can afford to do such swaps, gas mileage isnt really important to you.